PLANT SOCIETIES
In the long course of time in which plants have been accommodating
themselves to the varying conditions in which they are obliged to
grow, _they have become adapted to every different environment_.
Certain plants, therefore, may live together or near each other,
all enjoying the same general conditions and surroundings. These
aggregations of plants that are adapted to similar general conditions
are known as =plant societies=.
Moisture and temperature are the leading factors in determining
plant societies. The great geographical societies or aggregations
of the plant world may conveniently be associated chiefly with the
moisture supply, as: _wet-region societies_, comprising aquatic and
bog vegetation (Fig. 8); _arid-region societies_, comprising desert
and most sand-region vegetation; _mid-region societies_, comprising
the mixed vegetation in intermediate regions (Fig. 9), this being the
commonest type. Much of the characteristic scenery of any place is
due to its plant societies. Arid-region plants usually have small and
hard leaves, apparently preventing too rapid loss of water. Usually,
also, they are characterized by stiff growth, hairy covering, spines,
or a much-contracted plant body, and often by large underground parts
for the storage of water.
Plant societies may also be distinguished with reference to latitude
and temperature. There are _tropical societies_, _temperate-region
societies_, _boreal_ or _cold-region societies_. With reference
to altitude, societies might be classified as _lowland_ (which
are chiefly wet-region), _intermediate_ (chiefly mid-region),
_subalpine_ or _mid-mountain_ (which are chiefly boreal), _alpine_ or
_high-mountain_.
The above classifications have reference chiefly to great
geographical floras or societies. But there are _societies within
societies_. There are small societies coming within the experience of
every person who has ever seen plants growing in natural conditions.
There are roadside, fence-row, lawn, thicket, pasture, dune,
woods, cliff, barn-yard societies. _Every different place has its
characteristic vegetation._ Note the smaller societies in Figs. 8 and
9. In the former is a water-lily society and a cat-tail society. In
the latter there are grass and bush and woods societies.
[Illustration: FIG. 8.—A WET-REGION SOCIETY.]
=Some Details of Plant Societies.=—Societies may be composed of
_scattered and intermingled plants_, or of dense _clumps_ or _groups
of plants_. Dense clumps or groups are usually made up of one kind
of plant, and they are then called =colonies=. Colonies of most
plants are transient: after a short time other plants gain a foothold
amongst them, and an intermingled society is the outcome. Marked
exceptions to this are grass colonies and forest colonies, in which
one kind of plant may hold its own for years and centuries.
[Illustration: FIG. 9.—A MID-REGION SOCIETY.]
In a large newly cleared area, plants usually _first establish
themselves in dense colonies_. Note the great patches of nettles,
jewel-weeds, smart-weeds, clot-burs, fire-weeds in recently cleared
but neglected swales, also the fire-weeds in recently burned areas,
the rank weeds in the neglected garden, and the ragweeds and
May-weeds along the recently worked highway. The competition amongst
themselves and with their neighbours finally breaks up the colonies,
and _a mixed and intermingled flora is generally the result_.
In many parts of the world the _general tendency of neglected areas
is to run into forest_. All plants rush for the cleared area. Here
and there bushes gain a foothold. Young trees come up; in time these
shade the bushes and gain the mastery. Sometimes the area grows to
poplars or birches, and people wonder why the original forest trees
do not return; but these forest trees may be growing unobserved
here and there in the tangle, and in the slow processes of time the
poplars perish—for they are short-lived—and the original forest may
be replaced. Whether one kind of forest or another returns will
depend partly on the kinds that are most seedful in that vicinity and
which, therefore, have sown themselves most profusely. Much depends,
also, on the kind of undergrowth that first springs up, for some
young trees can endure more or less shade than others.
[Illustration: FIG. 10.—OVERGROWTH AND UNDERGROWTH IN THREE
SERIES,—trees, bushes, grass.]
Some plants _associate_. They grow together. This is possible largely
because they diverge or differ in character. Plants associate in two
ways: _by growing side by side_; _by growing above or beneath_. In
sparsely populated societies, plants may grow alongside each other.
In most cases, however, there is _overgrowth_ and _undergrowth_: one
kind grows beneath another. Plants that have become adapted to shade
are usually undergrowths. In a cat-tail swamp, grasses and other
narrow-leaved plants grow in the bottom, but they are usually unseen
by the casual observer. Note the undergrowth in woods or under trees
(Fig. 10). Observe that in pine and spruce forests there is almost no
undergrowth, partly because there is very little light.
On the same area the societies may _differ at different times of
the year_. There are spring, summer, and fall societies. The knoll
which is cool with grass and strawberries in June may be aglow with
goldenrod in September. If the bank is examined in May, look for
the young plants that are to cover it in July and October; if in
September, find the dead stalks of the flora of May. What succeeds
the skunk cabbage, hepaticas, trilliums, phlox, violets, buttercups
of spring? What precedes the wild sunflowers, ragweed, asters, and
goldenrod of fall?
=The Landscape.=—To a large extent the _colour of the landscape_
is determined by the character of the plant societies. Evergreen
societies remain green, but the shade of green varies from season to
season; it is bright and soft in spring, becomes dull in midsummer
and fall, and assumes a dull yellow-green or a black-green in winter.
Deciduous societies vary remarkably in colour—from the dull browns
and grays of winter to the brown greens and olive-greens of spring,
the staid greens of summer, and the brilliant colours of autumn.
The _autumn colours_ are due to intermingled shades of green,
yellow and red. The coloration varies with the kind of plant, the
special location, and the season. Even in the same species or kind,
individual plants differ in colour; and this individuality usually
distinguishes the plant year by year. That is, an oak which is maroon
red this autumn is likely to exhibit that range of colour every year.
The autumn colour is associated with the natural maturity and death
of the leaf, but it is most brilliant in long and open falls—largely
because the foliage ripens more gradually and persists longer in such
seasons. It is probable that the autumn tints are of no utility to
the plant. _Autumn colours are not caused by frost._ Because of the
long, dry falls and the great variety of plants, the autumnal colour
of the American landscape is phenomenal.
=Ecology.=—The study of the relationships of plants and animals to
each other and to seasons and environments is known as =ecology=
(still written _œcology_ in the dictionaries). It considers the
habits, habitats, and modes of life of living things—the places in
which they grow, how they migrate or are disseminated, means of
collecting food, their times and seasons of flowering, producing
young, and the like.
SUGGESTIONS.—One of the best of all subjects for school instruction
in botany is the study of plant societies. It adds definiteness
and zest to excursions. =7.= Let each excursion be confined to one
or two societies. Visit one day a swamp, another day a forest,
another a pasture or meadow, another a roadside, another a weedy
field, another a cliff or ravine. Visit shores whenever possible.
Each pupil should be assigned a bit of ground—say 10 or 20 ft.
square—for special study. He should make a list showing (1) how
many kinds of plants it contains, (2) the relative abundance of
each. The lists secured in different regions should be compared. It
does not matter greatly if the pupil does not know all the plants.
He may count the kinds without knowing the names. It is a good plan
for the pupil to make a dried specimen of each kind for reference.
The pupil should endeavour to discover why the plants grow as they
do. Note what kinds of plants grow next each other; and which are
undergrowth and which overgrowth; and which are erect and which
wide-spreading. _Challenge every plant society._